The molecular mechanisms that underlie endometrial differentiation are poorly understood. The long term objectives of this investigation are to elucidate the molecular mechanisms by which HOX genes regulate endometrial development and receptivity. HOXA10 and HOXA11 are transcriptional regulators that are each necessary for endometrial receptivity and implantation as demonstrated by targeted mutation. HOX genes typically assign segmental identity during embryogenesis, however we have identified the necessary role of persistent adult endometrial HOX expression in mice. Similarly, in women we demonstrated menstrual cycle stage dependent HOX expression and have begun to identify the cellular and molecular mechanisms of HOX gene action in this tissue. In the previous grant period we determined that sex steroids are the principal regulators of HOXA10 and HOXA11 expression in this tissue. We generated significant preliminary evidence that HOX genes regulate cellular ultra structure, regulate the expression of target genes, and that alterations in HOX signaling occur in women with deficient implantation. We hypothesize that HOX gene expression is altered in several human endometrial disorders. Additionally, we hypothesize that these deficiencies in endometrial function are a result of altered HOX gene expression and failed activation of their downstream target genes. Specific aim 1 will determine the spatial, cellular, and temporal expression of HOXA10 and HOXA11 in human endometrial disorders and then test causality in an animal model. HOX genes and associated cofactors regulate target genes and pathways in the attainment of endometrial receptivity. Accordingly, Specific aims 2 and 3 will characterize the molecular mechanisms by which HOX genes regulate their targets. Specific aim 2 will characterize the endometrial expression and mechanism of action of HOX cofactors, which are necessary for HOX gene function and specificity of target gene regulation. This specific aim will also test the DNA binding specificity and transcriptional regulatory activity of a novel HOXA10 specific cofactor identified in preliminary data. Specific aim 3 will use micro array technology to identify the target genes of HOX transcriptional regulation in the endometrium. These studies will further characterize the molecular mechanisms by which HOX expression is related to normal and pathologic endometrial development.